The processability of a synthetic polymer is often a compromise between the ease of processing and desired product properties. Processing routes conventionally applied in the polymer industry are injection moulding, extrusion and blow moulding. All these routes start from a melt of the polymer. Melt properties are mostly affected by the molecular mass of the polymer.
For a melt consisting of relatively low molecular mass polymers (Mw<Mc) there is a direct proportionality between zero-shear viscosity (η0) and molecular mass, whereas for a melt consisting of high molecular mass polymers (Mw>Mc) the viscosity depends much more strongly on the molecular mass (η0˜(Mw)3,4). Herein is Mw the weight averaged molecular mass and Mc the critical molecular mass, which is related to the shortest polymer chain length able to form an entanglement. This difference in viscosity of the two molecular mass regimes is due to the ability of long chains to entangle, which imposes a restriction on the flowability of a melt.
The motion of chains within a highly entangled melt is described by the reptation model introduced by De Gennes in J. Chem. Phys. 55, p. 572 (1971). In this model a chain within a melt moves in worm-like fashion through a virtual tube, which is delineated by entanglements formed by neighbouring chains. The time needed for a chain to renew its tube (reptation time), i.e. to change its position within the melt is also highly dependent on molecular mass (τ0˜Mw3). These fundamental restrictions make high molecular mass polymers rather intractable via conventional processing routes. On the other hand, final properties like tenacity, strength and wear improve with increasing molecular mass. Superior properties are-necessary to meet the requirements of demanding applications.
The discrepancy between intrinsic properties related to high values of molecular mass and insufficient product performance due to difficulties in processing is encountered in UHMW-PE as well as in other polymers of very high molecular mass. UHMW-PE is a linear grade polyethylene, as is high-density polyethylene (HDPE), but possesses a weight average molecular mass of at least 7, ·5*105 g/mol (according to ASTM D4020). Preferably the UHMW-PE has a weight average molecular mass of at least 3*·106 g/mol, because of excellent mechanical properties.
A major breakthrough in the processing of UHMW-PE was achieved in the early 1980s when solution spinning of UHMW-PE into high modulus/high strength fibres was introduced. In the process described in UK Patent 2,051,661 UHMW-PE is dissolved in a solvent at elevated temperature and the semi-dilute solution is spun into filaments, e.g. gel filaments, which filaments are subsequently before, after and/or during removal of solvent drawn to high drawing ratios (above 30), at temperatures close to but below the dissolution or the melting point. Thus obtained fibres possess a tensile strength of about 3 GPa and a tensile or Young's modulus of above 100 GPa.
A disadvantage of the thus obtained fibre is that this fibre always contains a certain residual amount of solvent. In general the amount of solvent present in solution- or gel-spun fibres is at least 100 ppm. An identical polymer sample made by processing, and thus crystallisation from the melt could not be drawn more than 5-7 times, resulting in the fibre possessing poor mechanical properties.
These results suggested that the density of the entanglements plays a prominent role in the process of drawing and obtaining fully aligned chains in the direction of drawing. The effect of entanglement density was confirmed by drawing experiments on single-crystal mats from UHMW-PE, as reported by T. Ogita et al. in Macromolecules 26, p. 4646 (1993). In the case of melt crystallised UHMW-PE, entanglements are trapped upon crystallisation and limit the extent to which the chains can be drawn. On the other hand, crystallisation of long molecular chains from the semi-dilute solutions leads to a much less entangled system and this enables these materials to be drawn below the melting temperature. It has always been believed that once a disentangled state of UHMW-PE-has been achieved, the formation of entanglements within the melt will be very slow, due to a long reptation time, and consequently one would be able to benefit from a disentangled state during processing. Experimental results however showed that highly disentangled solution crystallised films of UHMW-PE, which are drawable below the melting temperature lose their drawability immediately upon melting. This phenomenon has been associated with that of “chain explosion”, as experimentally assessed by P. Barham and D. Sadler in Polymer 32, p. 939 (1991). With the help of in-situ neutron scattering experiments they observed that the chains of highly disentangled folded chain crystals of polyethylene increase the radius of gyration instantaneously upon melting. Consequently the chains entangle immediately upon melting, which causes the sudden loss in processability and drawability once the sample has been molten.
These results showed that the fundamental restrictions resulting from the strong dependence of the zero-shear viscosity on molecular mass cannot be easily overcome. Simple disentanglement of the chains prior to melting will not lead to a less entangled melt and accordingly it cannot be used to improve the melt processability of UHMW-PE.
The objective of the present invention is to provide a process for the manufacture of a shaped part of ultra high molecular weight polyethylene (UHMW-PE) comprising melt processing, which part shows good drawability below its melting point.
According to the invention this objective is achieved with a process characterized in that UHMW-PE    a. Is annealed at a temperature between 130° C. and 136° C., preferably at about 135° C., for at least 1 hour;    b. is subsequently converted into a shaped part at a temperature above 142° C.; and    c. is then cooled down to a temperature below 135° C.
With the process according to the invention a shaped part of ultra high molecular weight polyethylene can be made by melt processing. The part thus formed is still highly drawable below its melting point, which indicates that, even though the UHMW-PE is processed in the melt, it still has a low entanglement density. A further advantage of this process is that a shaped part of UHMW-PE is made that contains no or very low amounts of residual solvent.